Migration: warn if the input code has unresolved imports.

The most likely cause is that the user has forgotten to run "pub get",
so we suggest that as a possible fix.

Fixes #40824.

Change-Id: I9c627017af08f9a851cadb46be8d9bf90a64ce78
Bug: https://github.com/dart-lang/sdk/issues/40824
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141823
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>

pkg/dartdev/test/commands/migrate_test.dart

@@ -16,6 +16,8 @@ void main() {
 bool _nnbdIsEnabled = false;
 
 void defineMigrateTests() {
+  final didYouForgetToRunPubGet = contains('Did you forget to run "pub get"?');
+
   TestProject p;
 
   tearDown(() => p?.dispose());
@@ -67,4 +69,22 @@ void defineMigrateTests() {
         contains('not found; please provide a path to a package or directory'));
     expect(result.stdout, isEmpty);
   });
+
+  test('pub get needs running', () {
+    p = project(mainSrc: 'import "package:foo/foo.dart";\n');
+    var result = p.runSync('migrate',
+        ['--server-path=${p.absolutePathToAnalysisServerFile}', p.dirPath]);
+    expect(result.exitCode, 1);
+    expect(result.stderr, isEmpty);
+    expect(result.stdout, didYouForgetToRunPubGet);
+  });
+
+  test('non-pub-related error', () {
+    p = project(mainSrc: 'var missing = "semicolon"\n');
+    var result = p.runSync('migrate',
+        ['--server-path=${p.absolutePathToAnalysisServerFile}', p.dirPath]);
+    expect(result.exitCode, 1);
+    expect(result.stderr, isEmpty);
+    expect(result.stdout, isNot(didYouForgetToRunPubGet));
+  });
 }

pkg/dartfix/lib/src/migrate/migrate.dart

@@ -158,12 +158,14 @@ class MigrateCommand extends Command {
           fileErrors.values.map((list) => list.length).reduce((a, b) => a + b);
       logger.stdout(
           '$issueCount analysis ${pluralize('issue', issueCount)} found:');
+      List<AnalysisError> allErrors = fileErrors.values
+          .fold(<AnalysisError>[], (list, element) => list..addAll(element));
       _displayIssues(
         logger,
         options.directory,
-        fileErrors.values
-            .fold(<AnalysisError>[], (list, element) => list..addAll(element)),
+        allErrors,
       );
+      var importErrorCount = allErrors.where(_isUriError).length;
 
       logger.stdout('');
       logger.stdout(
@@ -172,9 +174,14 @@ class MigrateCommand extends Command {
       if (options.ignoreErrors) {
         logger.stdout('Continuing with migration suggestions due to the use of '
             '--${MigrateOptions.ignoreErrorsOption}.');
-      } else if (!options.ignoreErrors) {
+      } else {
         // Fail with how to continue.
         logger.stdout('');
+        if (importErrorCount != 0) {
+          logger.stdout(
+              'Unresolved URIs found.  Did you forget to run "pub get"?');
+          logger.stdout('');
+        }
         logger.stdout(
             'Please fix the analysis issues (or, force generation of migration '
             'suggestions by re-running with '
@@ -371,6 +378,8 @@ the tool with --${MigrateOptions.applyChangesOption}).
     }
   }
 
+  bool _isUriError(AnalysisError error) => error.code == 'uri_does_not_exist';
+
   /// Parse and validate the user's options; throw a UsageException if there are
   /// issues, and return an [MigrateOptions] result otherwise.
   MigrateOptions _parseAndValidateOptions() {